1. Field of the Invention
The present invention relates to a pressure-sensitive breaker for a battery, and more particularly, to a pressure-sensitive breaker for a battery used to ensure safety when the internal pressure in a battery case rises.
2. Description of the Related Art
In a chargeable lithium ion secondary battery or the like, when any abnormality is caused by overcharging or overdischarging of the battery, a large quantity of gas may be generated due to decomposition, evaporation, or the like of an electrolytic solution, and the pressure inside the closed battery case may rise. This may deform and burst the battery case, or cause leakage of the electrolytic solution.
As a countermeasure thereto, a pressure-sensitive breaker for a battery is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-335272, in which overcharging or overdischarging of a battery can be prevented by cutting off the electric path of the battery in response to deformation of the battery case.
FIGS. 8 to 10 show the above-described conventional pressure-sensitive breaker for a battery. In the pressure-sensitive battery breaker, a ceramic substrate 21 is closely attached to a side face of a battery case 28a of a rectangular battery 28, such as a lithium ion secondary battery.
The battery 28 is covered with the battery case 28a on the periphery, as shown in FIG. 8. A first electrode 25 and a second electrode 26 project from the left side face of the battery case 28a in the figure, and a third electrode 27 projects from the right side face. Electromotive force is generated between the second and third electrodes 26 and 27.
The ceramic substrate 21 made of a fragile material and having a rectangular outline is placed on the upper surface of the battery case 28a. A conductive pattern 22 is formed by printing on the upper surface of the ceramic substrate 21, and a V-shaped groove 23 is formed across the full width on the lower surface of the ceramic substrate 21. A portion of the ceramic substrate 21 at the groove 23 is easily broken.
A pair of terminals 24 are provided to attach the battery 28 and the ceramic substrate 21 into a unit. Each of the terminals 24 is formed by bending an elastic metal plate of phosphor bronze, stainless steel, or the like, and comprises legs 24a and 24b bifurcated in elastic contact with the conductive pattern 22, a leg 24c in elastic contact with the lower surface of the battery 28, and a louver portion 24d in which the first or third electrode 25 or 27 projecting from the left or right side face of the battery case 28a is fitted. The terminals 24 are substantially angularly U-shaped overall.
When attaching the pressure-sensitive battery breaker to the battery 28, as shown in FIGS. 8 and 9, the ceramic substrate 21 is placed on the upper surface of the battery case 28a, and the terminals 24 are fitted thereon from the right and left sides in the figure by using an assembly jig (not shown), so that the battery 28 and the ceramic substrate 21 are pressed and clamped by the bifurcated legs 24a and 24b. Moreover, the first and third electrodes 25 and 27 are fitted into the louver portions 24d and are electrically connected to the conductive pattern 22 via the terminals 24.
That is, the ceramic substrate 21 is attached to the side face of the battery 28 by the terminals 24, and an electric path is formed between the first and third electrodes 25 and 27.
A description will be given of the operation of the conventional pressure-sensitive battery breaker with such a configuration. The battery 28 equipped with the pressure-sensitive battery breaker is incorporated in a portable device or the like (not shown), and the first and second electrodes 25 and 26 are used as power terminals of the portable device or the like.
When overcharging or overdischarging is performed, as described above, pressure inside the battery 28 rises, and the battery case 28a expands and deforms.
The ceramic substrate 21 is broken at the groove 23 due to deformation of the battery case 28a, as shown in FIG. 10, thereby cutting the conductive pattern 22.
The electric path between the first and third electrodes 25 and 27 is thereby cut off, the supply of electric power to the battery 28 is terminated, and expansion of the battery case 28a stops, whereby the battery 28 is prevented from bursting.
In a case in which such a conventional pressure-sensitive battery breaker is used in a low-profile and light-weight polymer lithium secondary battery or the like which has been recently sold commercially, since the battery case of the polymer lithium secondary battery is made of a flexible film formed of a laminate of aluminum foil and plastic, it is bent and deformed when the ceramic substrate 21 made of a fragile material is attached to the side face of the flexible battery case and tightly clamped by the terminals 24. This may crack the ceramic substrate 21 at the groove 23 and cut off the electric path between the first and third electrodes 25 and 27.
Furthermore, since the louver portions 24d of the terminals 24 are fitted on the first and third electrodes 25 and 27 when attaching the ceramic substrate 21 to the battery 28, a special assembly jig is required. This decreases assembly efficiency.
The present invention has been made in consideration of the above problems, and an object of the present invention is to provide a pressure-sensitive breaker for a battery with high assembly efficiency, which can be used in a polymer lithium secondary battery having a battery case formed of a laminated film or the like without any problem.
In order to achieve the above object, according to an aspect of the present invention, there is provided a pressure-sensitive breaker for a battery including a battery having a battery case for holding a power-generating element therein, a battery pack for accommodating the battery, and an element made of a fragile material, wherein a recess is formed at a position on the inner surface of the battery pack opposing a side face of the battery case, a first terminal is provided to be connected to an external electrode of the battery pack and a second terminal is provided to be connected to one electrode of the battery, a conductive portion is formed on the surface of the element, both ends of the conductive portion are connected to the first and second terminals so as to form an electric path between the first and second terminals, the element is deformed toward the recess and is broken due to bending of the side face of the battery case toward the recess caused when the internal pressure in the battery rises above a predetermined value, and the electric path is cut off by the breaking of the element.
In this case, the recess is formed on the inner surface of the battery pack, the electric path is formed of the conductive portion on the surface of the element, and both ends of the conductive portion are connected to the first and second terminals. The element is deformed toward the recess and is broken due to bending of the side face of the battery case toward the recess caused when the internal pressure in the battery rises above a predetermined value, and the electric path is cut off by the breaking of the element. Therefore, even if the battery has a low profile, when the side face of the battery case is bent, the element can be easily broken to cut off the electric path. This makes it possible to provide a high-performance and safe pressure-sensitive breaker for a battery which does not cause bursting of the battery, leakage of the electrolytic solution, and the like even when an abnormality occurs inside the battery.
Preferably, the element is supported over the recess so as to be stretched between shoulder portions disposed on both sides of the recess.
Since the battery case is deformed, the element is broken from the center on the shoulder portions, thereby reliably cutting off the electric path.
Preferably, the element is supported so that one end portion thereof is placed at one of the shoulder portions disposed on both sides of the recess and so that the other end portion is placed over the recess.
In this case, the element is cantilevered by one of the shoulder portions of the recess, and the other end portion placed over the recess serves as a free end. Accordingly, the free end can be pushed up to easily break the element even by a small amount of bending of the battery case.
Preferably, one of the first and second terminals has a U-shaped connecting portion formed by louvering at the end thereof, and the other end portion of the element placed over the recess is clamped by the U-shaped connecting portion.
Since the element and the terminal can be connected only by inserting the other end portion of the element into the connecting portion formed in one of the first and second terminals, it is possible to provide a pressure-sensitive breaker for a battery with high assembly efficiency.
A part of the first terminal or/and the second terminal may be embedded in the battery pack. This makes it possible to reduce the man-hours needed to mount the first terminal or/and the second terminal in the battery pack, and to thereby increase assembly efficiency.
The battery pack may have a through hole opposing a connecting portion between the first terminal or/and the second terminal, and the element. This makes it possible to connect the terminals and the element through the through hole by welding or by other means, and to provide a pressure-sensitive breaker for a battery with high assembly efficiency.
The through hole may be covered with a seal. This makes it possible to prevent dust and the like from entering the inner cavity from the outside.
Preferably, the battery and the battery pack are rectangular in outline, the recess is formed in a portion of the inner surface of the battery pack where the orthogonal line of the battery lies, and the element is placed on the orthogonal line. This makes it possible to lengthen the element to be stretched over the recess, and to thereby provide a high-performance pressure-sensitive breaker for a battery in which the element can be easily broken even by a small amount of expansion of the battery case.